In recent years, a technique for proliferating or maintaining various organs, tissues and cells which play different roles in a living body of an animal or a plant outside the living body has now been developing. To proliferate or maintain these organs and tissues outside the living body are called organ culture and tissue culture, respectively, and to proliferate, differentiate or maintain the cells separated from the organ or the tissue outside the living body is called cell culture. The cell culture is a technique in which the separated cells are proliferating, differentiating or maintaining in a medium outside the living body, and is indispensable for analyzing in detail functions and structures of various kinds of organs, tissues or cells in the living body. Also, the cells and/or the tissues cultured by the technique are utilized in various fields such as evaluation of pharmaceutical effects and toxicity evaluation of chemical substances, medicine, etc., mass production of useful substances such as an enzyme, a cell growth factor, an antibody, regenerative medicine which complements an organ, a tissue or a cell lost by a disease or a defect, breeding of plants, preparation of genetically modified crops, etc.
The cells derived from an animal are roughly divided into two of a floating cell and an adherent cell from their characteristics. The floating cell is a cell which does not require scaffolds for growth and proliferation, while the adherent cell is a cell which requires scaffolds for growth and proliferation, and almost all the cells constituting the living body are the latter adherent cells. As a method for culturing the adherent cells, the monolayer culture, the dispersion culture, the embedding culture, the microcarrier culture and the cell aggregate (sphere) culture, etc., have been known.
In particular, in recent years, accompanying with development in the field of the regenerative medicine, sphere culture has been attracted attention as a culture method which is closer environment to in the living body, and medium compositions or medium additives suitable for the culture have been variously reported (for example, see Patent Documents 1 and 2). Also, in the sphere culture, it has been considered that stimulation from the culture vessel (the substrate) is an important factor which influences the result of the culture, so that it has been required to culture the cells (in particular, a cell aggregate) at a three-dimensional environment or a complete floating condition without any stimulation from the culture vessel (for example, see Patent Document 3).